As illustrated in FIG. 23, a six-axis manipulator, which is a sort of an articulated robot, has a J1 axis, a J2 axis, and a J3 axis, which are joint axes that control an arm shaft mounted on a base. The J1 axis, the J2 axis, and the J3 axis are sequentially arranged from the base. In addition, the six-axis manipulator has a J4 axis, a J5 axis, and a J6 axis, which control wrist axes and are sequentially arranged from an arm portion to the top end of the wrist. In such a case, if joints angles θ1 to θ6 are given to the J1 axis to J6 axis, respectively, the XYZ coordinates of the position of an end effector (an effector; such as a welding device, attached to the top end of a robot arm) and the attitude angles of the end effector α, β, and γ (note that attitude angles α, β, and γ are in the form of Euler angles or the roll/pitch/yaw angles) can be obtained by calculating a solution to a forward kinematics problem. Conversely, if the XYZ coordinates of the position and the attitude angles α, β, and γ of the end effector are given, the joint angles θ1 to θ6 of the J1 axis to the J6 axis can be obtained by calculating a solution to an inverse kinematics problem.
As illustrated in FIG. 24, when the joint angle θ5 of the J5 axis is substantially 0 degree and if the end effector is attempted to move at a constant speed, the joint angle θ4 of the J4 axis and the joint angle θ6 of the J6 axis rapidly change. Accordingly, the J4 axis and the J6 axis need to rotate at high speed. However, the speed of each of the joint shafts of the manipulator cannot exceed the upper limit. If the manipulator is forced to operate so that the speed exceeds its upper limit, the end effector may step off a predetermined path or the end effector may vibrate. As described above, even if the position and the attitude angle of the end effector of the articulated manipulator is slightly varied, a joint shaft that is required to rotate at a significantly high rotational speed may appear and, therefore, the articulated manipulator may stop. If such stoppage of the articulated manipulator occurs, it is difficult to maintain the speeds of the joint shafts and the accuracy of the path along which the end effector should move at the same time. Thus, the speed of each of the joint shafts may be decreased to lower than the rotational speed required for the joint shaft to maintain the accuracy of the path. Alternatively, a safety mechanism may be activated, stopping the operation of the manipulator.
Accordingly, PTLs 1 to 3 describe a method for operating an end effector while preventing abrupt change in the operations of the wrist shafts and maintaining the speed of each of the wrist shafts within a limited speed when the speed of one of the wrist shafts exceeds the limited speed.